Spitzer observations of Lyman Break Spitzer observations of Lyman Break GalaxiesGalaxies

Dimitra Rigopoulou (Oxford,UK)J.-S. Huang, G. Fazio,(CfA, Harvard),& the IRAC team

C. Papovich, E. Egami (Arizona, MIPS)

Extreme Starbursts: Near and Far, 14-20 August, Lijiang, China

The observations

Deep IRAC & MIPS imaging (part of the IRAC GTOprogram)

IRAC: 3 hrs/pointingMIPS: slow scanning

2 deg x 10 arcmin Limiting flux densities (5 sigma)

(3.6 µm) : 0.5 µJy(4.5 µm) : 0.5 µJy(5.8 µm) : 2.7 µJy(8.0 µm) : 2.7 µJy(24 µm) : 60 µJy

Extended Groth Strip (EGS)

The EGS Lyman Break Galaxies sample

• Among 334 LBGs in the EGS area, 193 with spectroscopic redshfit (Steidel et al 2003)

• 244 are in the Spitzer EGS field

• ~200 are detected in the 3.6/4.5µm band

• ~50 are detected in the 5.8/8.0µm band

• 6 are detected in the 24µm band

R [3.6] [4.5] [5.8] [8.0] [24]

Rest-frame shifted and R-normalised SEDs

• Spitzer reveals the diversity of LBGs

• IRAC-bright LBGs have more dust? more massive?

• MIPS detections probes the rest—frame mid-infrared

U G R J K IRAC MIPS

Mag

(A

B)

Rest wavelength (microns)

Estimating stellar masses• Use Bruzual and Charlot (2003) code with• Padova 1994 tracks

• Exponentially decaying SFR α exp (-t / τ) where τ=0.05, 0.1,0.5,2.0, 5.0 Gyr (e-folding times) t = age• Constant SFR

• Solar metallicity

• Calzetti (2000) extinction law

Constant vs. exponentially decaying SF models

Huang et al. 2005

Huang et al. 2005

BC03 model fits

Rigopoulou et al. 2005

Blue LBGRed LBG

Constant vs. Exponentially decaying Mass estimates

Large scatter in [3.6] fluxes

4 magnitudes in [3.6]

1.5 magnitude in [K](Shapley et al. 2001)

[5.8] rest frame H-band

[3.6] rest frame I-band

Mid-IR colours of LBGs

Mid-IR colours of LBGs

[8.0] rest—frame K-band

Best tracer for mass

Massive LBGs have R-[3.6] >=3

A test – case: Westphal D49

In K-selected LBG sample of Shapley et al. (2001)

Constant SFR model U G R J K U G R J K IRAC Shapley et al. (1991) Our model: Age: 1139 Myr Age: 1350 MyrE(B - V) : 0.17 E(B - V): 0.35

Does extinction matter?

IRAC has “discovered” a new class of LBGs:

• luminous at [8.0] microns

• faint in the optical bands

• significant(?) dust extinction

• slightly older stellar pops

• …. massive, M > 1011 M

missing link to SCUBA galaxies?

SCUBA galaxies

Cold SED: Arp220

80—90% of sampleStarburst-like

Warm SED: Mrk 231

Egami et al. (2004)

Origin of the [24] emission

@ z=3 [3.6] — [8.0] [z’] – [K]blue for SBred for AGN

[8.0] – [24] [K] – [6.0]red for SBblue for AGN

Ivison et al. 2004

Saracco 04

Drory 04

Cole et al 00

This work

Number density as a function of redshift

= 0.7 , m = 0.3, H0 = 70)

Kauffmann99

Baugh03

Summary

• LBGs are detected in the mid-IR. Optical does not reveal full complexity.

• Large scatter in IRAC fluxes variations in mass, age, extinction

• Average SED young populations

• We detect more massive LBGs at z=3

than predicted by hierarchical models

(downsizing…..)